Gradual Acceleration?

Hello, and thank you in advance for your help
I am making a game that uses lots of the plane mechanics
But thr code provided to me only half works
it has a constant speed either going up or down

can anyome help me getting a gravity-like feeling of an acceleration while pointing down and/or up(stalling)?

You would want to do something like (this is a really lame example but hopefully you get the idea).

transform.Translate(transform.forward * speed * Time.deltaTime);
speed += Time.deltaTime * speedMod;

This way with each frame (or run of this code if you use a coroutine or other bounding factor) the speed that the object is moving at is increasing with each frame.

If you want to reduce the speed of motion you simple reduce the value of speed. Which can either be done gradually kind of like adding to it.